Seven transmembrane-spanning receptors (7TMRs or G protein-coupled receptors, GPCRs) represent the largest family of signal-transducing molecules known. 7TMRs convey signals for light and many extracellular regulatory molecules, such as, hormones, growth factors and neurotransmitters, that regulate every cell in the body. Dysregulation of 7TMRs has been found in a growing number of human diseases and 7TMRs have been estimated to be the targets of more than 30% of the drugs used in clinical medicine today. Thus, discovery of probes/drugs for 7TMRs is an important goal of biomedical research. We use high throughput screening (HTS) for small molecule ligands (SMLs) for 7TMRs with the receptors for thyroid-stimulating hormone (TSH-R) and thyrotropin-releasing hormone (TRH-R). During this year, we continued our development of these SMLs. 1) We previously reported on a SML agonist for TSH-R that could be used in an animal model to stimulate an increase in radioactive iodine uptake by the thyroid gland and in thyroid hormone secretion after oral administration. We had recognized that the SML was a racemic mixture of two enantiomers. We now have shown that the two enantiomers can be separated and that they exhibit different potencies in vitro and, more importantly, that one is far more effective than the other in vivo. We are currently following up on this observation to increase the bioavailability of the more active enantiomer. 2) We previously reported that a G protein-independent pathway, which uses beta-arrestin-1 as a signal transduction molecule, was important in the signaling by the TSH-R in bone precursor cells. We have now completed a high-throughput screen (in collaboration with NCATS) for SMLs that activate the TSH-R via a beta-arrestin-1 dependent, G protein-independent mechanism and found several lead compounds. We are currently working with chemists at NCATS to improve the pharmacological characteristics of these lead compounds. We think these compounds may be lead drugs for the treatment of osteoporosis in humans.